Protection of heat resistant alloys against carburization



Patented Oct. 6, 1936 UNITED sr Adrien @ambron, @ttawa, @ntarlo, Qanada No Drawing. Application December 26, 1934, Serial No. 259,232;

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This invention relates to the protection of reaction chambers, formed or heat resisting alloys, where these are used for carrying out reactions in which the metal is subjected to a carburizing atmosphere at high temperatures.

It has been found that when such chambers or tubes, which are normally resistant to air oxidation at temperatures of, say, 950 0., are used for purposes, such as the pyrolysis of hydrocar- 10 bon gases or the vapour-phase cracking of petroleum fractions, the metal becomes covered with a film of carbon. When air (is passed through the tube to remove the carbon film the metal liecomes coated with a loose deposit of a mixture of the oxides of the metals of the alloy. In the case of chromium-nickel or high chromium alloys the coating is of a brick red colour.

The formation of these coatings on the surfaces of the reaction tubes has a pronounced effect on the course of the reactions and reduces the rate of heat transfer through the walls as well as the capacity oi the tubes. The presence of the oxides causes the surfaces to become active in promoting carbon deposition and the increased rate at which the carbon deposits necessitates more ireuuent blowing oi the tubes with air to remove the carbon. For example, it has been reported that tubes made from an alloy containing 18% ohm-- mium and 8% nickel and used for the pyrolysis of hydrocarbon gases at about 900 C. required blowing with air every ill to 12 hours to remove the carbon deposit, and that the tubes failed after less than so home use, the failure being due to carburiration.

. The object of the present invention is to overcome these defects, to increase the useful life oi the tubes and to avoid the blocking oi the tube, as well as the lowering of the rate oi heat transfer through the wall of the tubes.

The invention contemplates preventing direct contact between the heat resistant alloys of such tubes and the carbon, so that the tubes may be heated alternately under carburizing and oxidizing conditions without decrease in their heat resistance or apparent alteration in the structure of the alloys. lfhis [is accomplished by covering the surface of the metal exposed to the reactions with a protective coating oi inert substances. The coating may be applied by brushing, spraying or 5 dipping, after which it is fused on the metal by heating to temperatures around 750 to 850 (3.

A thin continuous film of B20: forms such a protective coating. CraOa may be mixed with HaBOa in the proportions of one part oi the 55 Iormer with one to four parts oi the latter. Boric to the metal surfaces.

anhydride is the equivalent of boric acid. These materials are made up into a thin paste by suspending them in water or alcohol for application The Cl'aOa may be replaced in whole or in part by other refractory s oxides, such as BeO, MgO, A1203. When treating high chromium alloys, such as those containing 28% of chromium, the B203 may be replaced in whole or in part by VaOa and some of the ClcOr may be present in the form of chromite. 10

With alloys, such as those containing 18% ohm-- mium and 8% nickel, care must be taken not to subject them to too great and sudden changes of temperature, since this has a tendency to loosen the coating, owing to the relatively high 15 coefiicients of thermal expansion of such alloys. With high chromium alloys, such as those containing over 20% chromium, the so-coated metals may be heated to temperatures as high as 950C. and then cooled to room temperature without affecting the adherance or continuity of the coat= ing or altering its protective action.

The [invention has particular utility in the pyrolysis of hydrocarbon gases and in the vapour phase cracking of petroleum fractions at temperatures about VOW-950 C. The protective coating prevents the carburization of the metal of the reaction tubes and the results which follow such action, maintains the normal rate of heat conductivity of the metal and eliminates the cata= lytic efiect on. carbon deposition oi? the oxide deposits which term on unprotected metals of this character.

I claim:

1. Heat resistant chromium alloys having a 35 surface coating consisting of a continuous film consisting essentially of one of the group con= sisting of R203 and V205, said film being rused on the surface oi the alloy.

2. Heat resistant alloys having a surface coating consisting of a continuous film of a mixture oi one of the group consisting of B20: and V265 and one of the refractory group consisting of Cl'zOa, BeO, MgO and A1203, said film being fused on the surface of the alloy.

3. Chromium alloys having a coating consisting of a continuous film. consisting of B203 and CizOa, said film being fused on the surface of the alloy.

4. A method of protecting heat resistant ailoys against carburization during pyrolysis of hydro= carbon gases or vapours which comprises applying a continuous coating consisting essentially of B30: to the surface of the alloy and fusing the coating thereto. cs

leum fractions which comprises forming a mixture consisting essentially of CraOa and 310:. applyin: a continuous mm of the mixture to the alloy and heating the coated alloy at about 750 to 850 C. to fuse the film on the alloy.

ADRIEN CAMERON. 

